Water injection plasma arc cutting is used for cutting steel and non-ferrous metal plates for many applications. The plate to be cut is usually supported on a horizontal table and may be covered by two or three inches of water. The cutting torch is mounted above the table and projects a swirling jet of hot plasma of confined area through a metal nozzle onto the plate. The plasma is an ionized gas containing about equal numbers of positive ions and electrons. Water is pumped into the plasma torch to cool it and then is injected against the plasma for further constriction of the arc. A drawback of the process is that torch nozzle life is short. The narrow water passages in the nozzle tend to distort, which not only reduces the cooling efficiency of the water but causes alteration in the flow pattern of the swirling gas. Water hardness has been blamed for this difficulty. Where water hardness is as high as 35 grains per gallon, measured as calcium carbonate (CaCo.sub.3), nozzle life of only a few minutes has been observed. Prior to my invention to be described hereinafter water injection plasma arc cutting torch users have been advised to soften the water supplied to the plasma torch, even when the hardness is as low as 7 grains per gallon. However, such treatment produces only moderate improvement, if any, in torch operation.